Linux: Correctly count the number of open file descriptors.

base/files/dir_reader_fallback.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_FILES_DIR_READER_FALLBACK_H_
 #define BASE_FILES_DIR_READER_FALLBACK_H_
 
 namespace base {
 
 class DirReaderFallback {
  public:
   // Open a directory. If |IsValid| is true, then |Next| can be called to start
   // the iteration at the beginning of the directory.
   explicit DirReaderFallback(const char* directory_path) {}
 
   // After construction, IsValid returns true iff the directory was
   // successfully opened.
   bool IsValid() const { return false; }
 
   // Move to the next entry returning false if the iteration is complete.
   bool Next() { return false; }
 
   // Return the name of the current directory entry.
-  const char* name() { return 0;}
+  const char* name() { return nullptr;}
 
   // Return the file descriptor which is being used.
   int fd() const { return -1; }
 
   // Returns true if this is a no-op fallback class (for testing).
   static bool IsFallback() { return true; }
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_DIR_READER_FALLBACK_H_